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Waterstop Calculator

Joint length gives waterstop length, area and weight.

Quick answer

With the default inputs: Total joint length 120 · Waterstop width 300 · Steel waterstop thickness 3 etc.Waterstop purchase length 132.0 m

Edit the inputs below to recalculate.
JointThk TWaterstopWidth W
The strip sits mid-joint with half its width embedded each side; displacement is the leading cause of leaks.

Inputs

Sum of all joints needing waterstop — slab, walls, roof
300 mm typically, half embedded each side
3 mm typical, 4-5 mm on critical work
%
100 mm laps per length plus cutting, usually 8-15%

Result

Waterstop purchase length
132.0m
Net joint length120.0m
Lap and waste10%
Purchase length132.0m
Surface area39.60m²
Steel weight per metre7.07kg/m
Total steel weight932.6kg
6 m lengths required22bars
Steel per 100 m of joint707kg

Length is the actual joint length times a lap and waste allowance. Both rubber and steel waterstop need laps: rubber is heat- or adhesive-spliced with at least 100 mm per joint; steel is welded with at least 50 mm full-penetration lap. Steel weight is width × thickness × 7850 kg/m³ — 300×3 mm gives 7.07 kg/m, worth memorising since it is bought by the tonne and installed by the metre. The strip must sit in the middle of the joint, half embedded each side, clamped with fixing bars so it cannot shift during the pour; displacement is the leading cause of leaks.

Formula

Purchase lengthL = joint length × (1 + lap and waste)
Surface areaA = L × width
Steel per metreg = width × thickness × 7850
Total steelG = L × g
6 m lengthsn = ⌈L ÷ 6⌉
120 m of joint with 300 mm wide, 3 mm steel waterstop and 10% laps: 132.0 m to buy, 39.60 m² of strip, 7.07 kg per metre, 932.6 kg total and 22 lengths of 6 m.

How to work it out

  1. Count the total joint length: horizontal construction joints at slab-to-wall by wall perimeter, vertical joints by wall height, and post-cast strips by running metre.
  2. Purchase length = joint length × (1 + lap allowance). Rubber waterstops lap 100-150 mm and must be hot-vulcanised; steel waterstops lap 50-100 mm and are welded.
  3. Steel waterstop weight = length × width (m) × thickness (mm) × 7.85, so 300 × 3 is 7.065 kg/m.
  4. Rubber waterstops are bought by the metre, commonly 300 × 8 and 350 × 10 (width × thickness); count embedded and surface-applied types separately.
  5. Add fixings: waterstops are tied to the reinforcement or held with purpose-made clamps at 0.5-1.0 m centres — a line that is routinely forgotten.

Where it goes wrong

Material and cost

Rubber waterstop 300 × 8 costs 25-50 CNY/m, steel-edged rubber 60-120, steel waterstop 300 × 3 35-55, swellable strip 15-30. Welding 10-20 CNY/m.

On site

Basis

FAQ

Why 3 mm plate rather than thinner?
It must weld without burning through and resist distortion. At 1-2 mm the poker bends it and rebar crushes it, opening a gap against the concrete that becomes a leakage path. 3 mm is standard, clamped with φ8 or φ10 bars at about 500 mm centres. Critical work — high water table, deep basements, grade-1 waterproofing — uses 4-5 mm. Width is normally 300 mm: narrower gives too little anchorage, wider is wasteful and hard to fix.
What happens if the waterstop is displaced?
It leaks, and it is nearly impossible to fix. The strip works by lengthening the seepage path: water has to travel around the embedded half, and the pressure is lost along that longer route. Shift it to one side and the other side is left with only a thin cover, the path shortens and the barrier fails. Remedial work means injection or chasing and resealing from the water face. So clamp it firmly, pour evenly both sides and never point the poker at it.
Rubber or steel waterstop?
Rubber or plastic strips flex with settlement and movement, so they suit movement and settlement joints; but they age, dislike heat and are attacked by oils. Steel is stiff, durable and weldable — right for construction joints (not movement joints, since it cannot accommodate displacement), especially horizontal joints in basement walls and slabs. Swelling strips absorb water and expand to seal, used as a secondary measure in construction joints, never as the primary barrier.

Cite this page

Use this format in reports, method statements or coursework. The link is permanent.

Building Calculator. Waterstop Calculator. 2026-10-25. https://jianzhujisuan.com/en/waterstop

Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.